We have continued our studies on the cytosolic phosphoprotein, Prosolin, which was discovered in this laboratory. It is a major cytosolic protein of proliferating human lymphocytes and promyelocytic cells, but is not expressed in resting peripheral blood lymphocytes. We found that prosolin is an important early target of the activities associated with T-cell receptor (TCr) activation, indicating that prosolin phosphorylation serves to transmit the signal generated by TCr occupancy in the proliferating normal lymphocyte. Prosolin phosphorylation induced by TCr activation or by other treatments is invariably followed by rapid, but temporary, down regulation of DNA synthesis in normal proliferating lymphocytes. This may represent a normal regulatory aspect of the immune system. The rapid phosphorylation of prosolin that follows TCr activation is complex, resulting in 4 phosphorylated forms of the molecule containing variable numbers of phosphorylated serine residues. Evidence suggests that more than one protein kinase is involved in this complex phosphorylation event, and that one of these activities may be deficient in T-cell leukemia cells with the result that some of the phosphorylated forms of prosolin do not appear in such cells. We have cloned and sequenced the prosolin cDNA from normal human peripheral blood lymphocytes and have identified various potential phosphorylation sites.